1. Field of the Invention
This invention relates to detectors and, more particularly, to the biasing of a linear array of photoconductive detectors.
2. Background Art
The use of photoconductive detectors for measuring radiation is well-known. Because of their high sensitivity, photoconductive detectors such as PbS and PbSe are particularly effective in measuring infrared radiation. Detection of infrared radiation is used by the military for tracking warm vehicles and in night vision devices, is used by medical instrument manufacturers for measuring glucose and other body constituents in a noninvasive manner and is used by scientific instrument manufacturers for measuring chemical composition and structure.
In general, the resistance of the photoconductive detector changes when the radiation falls on its surface. Resistance changes can be measured as an electrical signal change and the intensity of the detected radiation can be estimated by the magnitude of resistance change.
Photoconductive detectors typically require a bias current or voltage to operate, such as a direct current bias. The sensitivity of the detector is proportional to the magnitude of the applied bias. It is preferred to supply a high bias to such a detector to increase its sensitivity and to overcome the noise of the electronics associated with the detector in an overall detection system.
The bias can be delivered to the detector in various manners, including a voltage divider, a constant current or a constant voltage. FIG. 1A shows a known voltage divider arrangement in which a detector 2 is attached at one end to a bias electrode 4 and is connected at its other end to a load resistor 6 which is grounded. The voltage across the load resistor 6 is supplied through capacitor 8 to amplifier 10 which generates an output signal related to the intensity of the radiation 12 impinging upon the detector 2. In the voltage divider arrangement, the incident radiation 12 modulates the resistance of the detector 2 and, consequently, changes the current flowing through the detector 2 and the load resistor 6. These current changes are converted to voltage changes across the load resistor 6. The AC components of the voltage changes are passed through capacitor 8 to the amplifier 10 for further processing.
FIG. 1B shows a constant current mode of operation in which the detector 2 is connected to the bias electrode 4 and to a constant current source 14. As discussed above, the voltage drop across the detector 2 changes with incident radiation 12 and these voltage changes are coupled to amplifier 10 through capacitor 8.
A constant voltage mode of operation is shown in FIG. 1C. In this mode of operation, radiation induced resistance changes in the detector 2 are quantified by measuring the current flowing through the detector 2. This is typically accomplished by direct coupling of the detector 2 to the input of a current-to-voltage converter, often identified as a "transimpedance amplifier". The transimpedance amplifier shown in FIG. 1C includes operational amplifier 16 having one input terminal grounded and the other input terminal connected to the detector 2. Feedback resistor 18 extends between the output terminal of operational amplifier 16 and its input terminal receiving the output of the detector 2. The detector 2 is also connected to the bias electrode 4.
The bias voltage applied to a photoconductive detector also causes current to flow in the absence of incident radiation. This current, referred to as the "dark current", is usually large when compared to the current changes resulting from incident radiation. The detection of the small, radiation related signal, which is added to the large, dark signal, is often difficult. A typical arrangement for detecting the radiation related signal is shown in FIG. 2. Radiation from a radiation source 20, which could be a source of infrared radiation, passes through and is modulated by a modulator 22, such as a rotating slotted chopper disk, and then impinges upon a photoconductive detector 24. The arrangement shown in FIG. 2 is a constant voltage mode of operation in which a DC bias voltage is applied to the detector 24 from bias voltage source 26 and the output of the detector 24 is passed through a transimpedance amplifier 28 as discussed above in connection with FIG. 1C. The detector 24 converts the modulated radiation 30 into an electrical signal and the transimpedance amplifier 28 amplifies both the AC signal associated with the radiation 30 and the associated dark signal. Modulated detection, or preferably synchronous detection, is used to overcome the problem associated with the dark signal. The AC component of the signal generated by the transimpedance amplifier 28 is separated from the DC dark signal by coupling capacitor 32. The resulting AC component from the coupling capacitor 32 can be rectified synchronously with an external reference (not shown) derived from the modulator 22. The rectified signal from coupling capacitor 32 is passed through low pass filter 34, which can also include amplification, to reduce noise. The filtered signal is then digitized by an analog-to-digital converter 36 and the resultant digital signal can be used as desired.
It is common to use a plurality of photoconductive detector elements in the form of a linear array to measure radiation across a spectrum of wavelengths. Each detector element is responsive to and detects a particular wavelength, or band of wavelengths, of radiation. As shown in FIG. 3, a plurality of elongated photoconductive detector elements 40-46, also referred to as pixels, is shown mounted on a common substrate 48. Regardless of the type of biasing arrangement used, each element or pixel of the array will have one terminal, referred to as the input terminal, connected to a common electrode associated with the biasing source and another terminal, referred to as the output terminal, connected to the measuring scheme, which also provides a bias return path. As a result, the output terminals of the detector elements constitute a majority of the wiring and packaging feed-throughs from the substrate to the remainder of the system in which the substrate having the linear detector array is mounted. Rather than distribute the bias source along only one side of the detector array, i.e., with all of the input terminals of the detector elements aligned along one side of the substrate and with all of the output terminals of the detector elements aligned along the other side of the substrate, it is typical to distribute the input and output terminals of the detector elements evenly on both sides of the linear array structure as shown in FIG. 3. This configuration permits the design to be arranged such that the packaging is symmetrical and the interconnections are simplified. As can be seen in FIG. 3, the measurement devices, here transimpedance amplifiers 50-56, connected to the output terminals of detector elements 40-46, respectively, are evenly distributed on opposite sides of the substrate 48, rather than all being positioned on one side. Typically, odd detector elements in the array have their output terminals on one side of the substrate 48 while even detector elements in the array have their output terminal on the other side of the substrate 48. Such a configuration is often referred to as an "interdigitated" or "interlaced" arrangement. The arrangement shown in FIG. 3 is a constant voltage mode of operation, including a bias voltage source 58 supplying a bias voltage V.sub.b to the input terminal of each detector element 40-46 through a single bias electrode 60 on the substrate 48, and including a transimpedance amplifier 50-56 connected to the output terminal of each detector element 40-46. The single bias electrode 60 extends on the substrate along the length of the array and on each side of the detector elements. However, other biasing arrangements can also be utilized.
The interdigitated design shown in FIG. 3 results in potential differences between neighboring detector elements. This is shown in FIG. 3 where, for example, detector element 40 has the bias voltage V.sub.b applied to its input terminal while the immediately adjacent detector element 41 has a zero voltage at its output terminal which is immediately adjacent the input terminal of detector element 40. The opposite situation is shown at the opposite ends of detector elements 40 and 41 where the bias voltage V.sub.b is applied to the input terminal of detector element 41 which is adjacent the output terminal of detector element 40 having a zero voltage thereupon. The difference in voltage between the ends of adjacent detector elements 40 and 41 is the bias voltage V.sub.b. In the typical microcircuit arrangements in which these linear detector arrays are configured, the distance between the adjacent detector elements can be as small as 0.001 inch in a medium resolution array. Even a relatively low bias voltage can result in a high electrical field between the adjacent ends of adjacent detector elements. This high electrical field can encourage the migration of surface contaminants to these areas and create leakage currents therebetween. This configuration results in an increase in noise and instability and, with time, may lead to the failure of the array.
It is, therefore, an object of the present invention to minimize these problems of the prior art interdigitated linear arrays and minimize or eliminate the potential difference and resulting electric field between adjacent detector elements in the array while preserving the small, compact design of a microcircuit arrangement.